Could one of the most tragic and shocking moments in NASA’s history have been prevented? Go inside the heartbreaking loss of the Space Shuttle Challenger, from those who were there.

The 1993 siege on the Branch Davidian compound in Waco, Texas was the deadliest single event in the history of the ATF. Hear from two agents who were wounded, a shell-shocked reporter, a law enforcement officer, Koresh's would-be lawyer, the coroner, and the FBI agent who oversaw the 51-day standoff until the compound burned down with many of the Davidians still inside.

The most prestigious race in the country is also Boston's most popular event. Crowds turn out by the thousands to cheer on the runners of the Boston Marathon.

What began as a calm spring morning in Oklahoma City erupted into the largest terrorist act our country had ever experienced. The truck bomb left by Timothy McVeigh at the Alfred P. Murrah Federal building killed 168 people, and altered the lives of hundreds more.

The largest terror attack in our nation's history left images seared into our conscience that will never be forgotten. But for those caught in the middle, the memories are as strong today as they were in 2001.